Antenna tuner units (ATU) are currently being considered for use in mobile terminals used for mobile radio communications. One purpose of an ATU is to match the impedance between a power amplifier (PA) and an antenna, thereby maximizing total radiated power (TRP). Another purpose of an ATU is to increase reception of radio signals by maximizing total isotropic sensitivity (TIS) for the antenna, especially since the antenna can experience large voltage standing wave ratio (VSWR) changes. For example, an antenna's input impedance is one parameter that can be affected by changes in a user's body placement versus the antenna. Certain body placements relative to the antenna will result in decreased radiated power due to a relatively large amount of power being reflected off the user's body, thus limiting the antenna's TRP. When the antenna is used for signal reception, other body placements relative to the antenna will reduce the TIS, resulting in poor receiver performance.
Presently, ATUs for cellular applications or mobile Internet devices (MIDs) that use third generation (3G) or fourth generation (4G) cellular systems are dual purpose ATUs. These ATUs are dual purpose in that they are used for impedance matching in both a transmit path and a receive path between a transceiver interface and a communicatively coupled antenna. These dual purpose ATUs require tuning elements that can handle large voltages due to the relatively large transmit powers involved. For example, dual purpose ATUs must be able to withstand a 6:1 VSWR. At a +33 dBm output power radiated from an antenna, a 6:1 VSWR requires a peak-to-peak voltage of 70 Vpk-pk. Micro-electromechanical systems (MEMS) switches having a large voltage drive have been developed to handle such large voltages. A disadvantage of MEMS switches is cost. Solid state switches using silicon-on-sapphire (SOS) or silicon-on-insulator (SOI) can also be used to handle large peak-to-peak voltages. However, at least fourteen cascode switches per switch branch must be used in order to handle 70 Vpk-pk. Thus, a die size for dual purpose ATUs must be relatively large, on the order of 3 mm2. Due to such a relatively large die size, SOI technology may be required, which further increases cost. In a further complication, dual purpose ATUs in some cases requires coupling and detector circuitries to calculate optimum tuning settings.
Increasingly, there are wireless data applications for which a data rate is higher in a downlink direction than an uplink direction. As a result, there are asymmetrical data requirements between a receiver and a transmitter. These asymmetrical data requirements allows for a receive only ATU with an integrated switch that has significantly relaxed voltage handling requirements. As such, switch drivers, control circuitry, and programming can be reused to provide a low cost solution for wireless systems.
FIG. 1 is a block diagram of a related art diversity antenna system 10 that includes a related art ATU 12 having a receive only (RX) tuner circuit 14 that is usable to tune an RX antenna 16. The RX antenna 16 can be a diversity/multiple-input-multiple-output (MIMO)/integrated mobile broadcast (IMB) type antenna. The ATU 12 also includes an integrated RX switch 18 that selectively couples the RX tuner circuit 14 to the RX antenna 16 through an electrostatic discharge (ESD) protection circuit 20. An integrated tuner switch 22 selectively couples the RX tuner circuit 14 to a transceiver interface 24 through RX only filters 26.
A power amplifier (PA) circuitry 28 is coupled between the transceiver interface 24 and a single pole multiple throw (SPxT) switch 30 through duplexers 32. The SPxT switch 30 selectively couples a transmit (TX) antenna 34 to the duplexers 32 as well as to LTE-TDD RX band filters 36. An antenna matching and ESD circuit 38 is coupled between the SPxT switch 30 and the antenna 34. The PA circuitry 28 includes PA stages 40 and may include directional couplers 42 and TX switches 44. A switch mode power supply (SMPS) 46 is coupled to a battery 48 that provides power to the PA circuitry 28.
FIG. 2 is a block diagram of a related art TX only/RX only antenna system 50, wherein an RX antenna 52 is only usable for receiving signals and a TX antenna 54 is only usable for transmitting signals. As such, the duplexers 32 (FIG. 1) are replaced by TX only filters 56. A third antenna (not shown) could be added to provide RX diversity.
The related art diversity antenna system 10 (FIG. 1) and the related art TX only/ RX only antenna system 50 both need a broadband receive only tuner that cover bands that range in frequency from 728 MHz up to 2690 MHz in order to cover all of the bands of the third generation partnership project (3GPP). FIG. 3 is a table that provides frequency information for twenty operating bands of 3GPP. A first column of the table provides the number of each 3GPP band. A second column provides uplink (UL) band frequencies for a base station (BS) receive operation and a third column provides UL band frequencies for a user equipment (UE) transmit operation. In particular, the second column lists UL low frequencies (FUL LOW) and the third column lists UL high frequencies (FUL HIGH). A fourth column and a fifth column of the table provide downlink (DL) band frequencies for a base station (BS) transmit operation and a user equipment (UE) receive operation, respectively. In particular, the fourth column lists DL low frequencies (FDL LOW) and the third column lists DL high frequencies (FDL HIGH). A sixth column provides a duplex mode for each 3GPP band. The duplex mode is frequency division duplex (FDD) for all bands listed except for bands 15 and 16, which are reserved.
FIG. 4 is a circuit diagram of a related art broadband tuner circuit 58 that has a possibility of being integrated with an RX RF switch 60. For exemplary purposes, the RX RF switch 60 is a single pole four throw (SP4T) type having two low-band outputs 62 and two high-band outputs 64. A single reactance element 66 is coupled between a switch input 68 and an RF input 70. An antenna 72 is coupled to the RF input 70. The antenna 72 is a diversity MIMO type antenna. A first variable capacitive element 74 is coupled between the switch input 68 and ground GND. A second variable capacitive element 76 is coupled between the RF input 70 and ground GND. The first variable capacitive element 74 and the second variable capacitive element 76 combined with the single reactance element 66 forms a pi-network. However, the related art broadband tuner circuit 58 does not provide enough broadband tuning to cover the 3GPP bands because the single reactance element 66 presents a relatively large reactance at the highest frequencies of the 3GPP bands.
FIG. 5 is a circuit diagram of another related art broadband tuner circuit 78 that includes a low band tuner 80 and a high band tuner 82. The low band tuner 80 has a first reactance element 84 coupled between an RF input 86 and an first switch input 88. The low band tuner 80 further includes a first tunable capacitive element 90 that is coupled between the RF input 86 and ground GND, and a second tunable capacitive element 92 that is coupled between the first switch input 88 and ground GND. The high band tuner 82 further includes a second reactance element 94 coupled between the RF input 86 and a second switch input 96. A third tunable capacitive element 98 that is coupled between the RF input 86 and ground GND, and a fourth tunable capacitive element 100 that is coupled between the second switch input 96 and ground GND. However, the broadband tuner circuit 78 is deficient in that the first reactance element 84 requires a relatively large inductance value and the second reactance element 94 is needed to tune high-band frequencies. Another deficiency is that the low band tuner 80 will load the high band tuner 82 during high band operation. A similar deficiency occurs during low band operation. However, during low band operation, it is the high band tuner 82 that loads the low band tuner 80. Both of these loading deficiencies are caused by having the low band tuner 80 and the high band tuner 82 share the RF input 86 as a common node.
In spite of the difficulties mentioned above ATUs are presently being considered for MIDs in order to enhance the TRP/TIS of mobile terminals for the purpose of providing better high speed data operation. However, presently considered ATUs are designed to adjust impedance matching between an antenna and a radio interface to provide tuning for both a transmitter circuit and a receiver circuit. Thus, due to the large transmitter power, a relatively expensive dual purpose ATU solution is most often deemed necessary. However, a user usually operates a mobile terminal in a high speed data downlink mode. Moreover, with the advent of 4G mobile terminals like long term evolution time division duplex (LTE-TDD), which have higher modulation bandwidth and higher downlink data rates, the quality of downlink performance is critical. As a result, improvements in TIS are more critical than improvements in TRP. Therefore, what is needed is an ATU that includes a receive only tuner circuit for a mobile terminal that has increased TIS along with reduced cost of implementation.